Can you please click F12 in the browser and give us a screenshot of any errors shown in the JavaScript console? Thank you.

(That's a template snippet, but F12 usually gives us some good hint. :) )

I don't see anything in the console or network requests that is failing.

I was able to reproduce it in the following way:
1) Open the email templates screen of a survey with attachments, with or without errors.
2) Click on an attachment to open KCFinder, and close it
3) Press the Back button of the browser

Considering that it only happens after you have opened the KCFinder and apparently it hangs loading a gif, it must be something related to a loader that must be set the first time you open the kcfinder and it must remain set.

@gabrieljenik can you take this and make it a priority? It seems pretty bad to me.

Well, for me, pretty bad would be not allowing people to take surveys :)
This is not good, obviously, but many things are still working even with this bug :)